1. Field of the Invention
The present invention relates to methods and apparatus for improving the performance of a Sagnac-type fiber optic gyroscope. More particularly, the invention pertains to the improvement of scale factor stability in an open loop gyroscope and reduction of zero point drift and random walk in a closed loop gyro.
2. Description of the Prior Art
The present invention relates to fiber optic interferometers such as fiber optic gyroscopes for determining rotation rates, wherein light from a (preferably stabilized) light source such as a laser diode (LD) or a superluminescence diode (SLD) is irradiated, after two beam splittings, with like light intensities into the two ends of a fiber coil. After having passed through the coil (usually of polarization or non-polarization maintaining monomode fiber), the two light portions are superimposed to interfere at the second beamsplitter which constitutes the "main" beamsplitter. Upon return through a polarizer, optimally half the portion of the interfering light is decoupled at the first beamsplitter located near to the light source. This decoupled light is applied to a photodetector whose output signal, after amplification and A/D conversion, is subjected to frequency analysis and signal processing to obtain a rotation rate signal.
Fiber optic interferometers, particularly rotation rate sensors possessing this basic structure, are known in both open loop and closed-loop configurations. It is known to phase-modulate the light counterpropagating within the fiber coil so that the working point for signal measurement is always in the characteristic range of maximum measuring signal change per rotation rate change to increase accuracy of measurement. Modulation may be periodic or according to a statistical scheme.
FIG. 2 is a schematic diagram of a known open loop Sagnac-type fiber optic gyroscope configuration. Light, from a source 1 such as a super luminescent diode (LSD) that is stabilized in intensity and wavelength, is transmitted along a fiber path to a first beamsplitter 2, through a polarizer 3 and to a second "main" beamsplitter 4. The component beams produced by beam splitting enter the two inputs/outputs of a fiber coil 6 from the two inputs/outputs remote from the light source 1. Thereafter, a depolarizer 5 and a phase modulator 7 are provided between the inputs/outputs of the main beamsplitter 4 and the inputs/outputs of the fiber coil 6. In the event that non-polarization maintaining monomode fiber is employed, the depolarizer 5 acts to insure interference of the light. Such apparatus is omitted when polarization-maintaining monomode fiber is employed.
The phase modulator 7 is activated by a modulation oscillator 8 for periodically (or, according to a known quasi-stochastic modulation method) shifting the phase of the light provided for signal processing to a working point of maximum sensitivity of rotation of the fiber coil 6 about its axis. The beams interfering in the main beamsplitter 4 after having passed through the fiber coil 6 pass through the polarizer 3 with an optimum portion (one half) conducted through the first beamsplitter 2 to a photodetector 9 whose output signal is amplified and filtered in a predetermined way at 10, then digitized at 11 and subsequently subjected to frequency analysis at 12 and signal analysis at 13 to obtain the rate of rotation signal .OMEGA..
The accuracy of a fiber optic gyroscope--particularly in open loop configuration--depends, inter alia, on scale factor stability. Such scale factor and its stability are dependent, among other things, on the properties of the light source 1. In order to stabilize the light source 1 with respect to its zero point and wavelength variations despite the influences of such factors as the environment, fluctuations due to aging and fabrication, considerable efforts have been exerted as taught by DE 40 37 118 C1, DE 38 05 915 C2 and EP 0 611 950 B1, for example.
Despite such efforts, it has been found that such efforts for stabilizing the light source 1 and for compensation of light source wavelength variations respectively are inadequate to maintain scale factor sufficiently stable--insofar as it is influenced by the light source 1 when a specific measuring accuracy is required.
In a closed-loop configuration, basically two influencing magnitudes cause special accuracy problems. These are zero point drift (bias drift) and signal noise (random walk) in the measuring signal. Such factors are caused mainly by the electrical control of the light source and by crosstalk of the modulation voltage into the light source electronics at the I/O chip.